Several devices are currently known which monitor the presence of objects in a determined zone, such as the one described in U.S. Pat. No. 5,396,218 comprising a master card and at least one slave card or transponder, designed to be placed in the object to be monitored. The cards are adapted to exchange information between them through radio-frequencies, wherein the master card forms the central control system including an alarm circuit which is activated when the slave card leaves the field of protection.
German Patent No. 3,618,416 relates to an electronic device for monitoring luggage or other objects, comprising a central electronic unit exchanging encoded information with secondary units or transponders, that can be placed in the objects to be monitored or made integral with the latter. The central unit is programmable according to the number of secondary units, the level of reception, or other parameters, and transmits call signals to the secondary units responding to the central unit that trigger an alarm according to the signals received.
However, in these devices, the electronic circuits of the secondary units are permanently activated, which brings up the problem of power consumption and therefore of autonomy, in particular when small-sized secondary units, or transponders, are used with signal transmissions at high frequencies, higher than approximately 400 MHz, requiring a considerable power consumption to obtain a homogenous covering of transmissions/receptions within a volume of several meters of a spherical or delta type. On the other hand, these devices have an integrated power supply that can be neither recharged nor replaced. Besides, these devices oblige the user to activate and deactivate all transponders if he wishes to stop or re-activate the system, when he is at his working place or at home or some other place not requiring protection, which is inconvenient, since this intervention has to be performed several times a day.
On the other hand, these devices do not provide signaling in case a supernumerary unit with the same encoding is inserted into the protection field of the device for the purpose of maleficent substitution of a protected secondary unit.